1. Field of the Invention
The present invention relates to lithographic processing of semiconductors and, in particular, to a method of detecting and measuring edges of features using imaging apparatus.
2. Description of Related Art
A general problem in the field of lithographic processing of semiconductors is to accurately measure the dimensions of an object which has been imaged by some apparatus. As used herein, imaging apparatus includes optical microscopes and scanning electron microscopes (SEMs). An example of such problem in lithographic processing of semiconductors is the measurement of the line width of a resist line on a silicon wafer, as would be formed during the fabrication of integrated circuits (ICs), with an optical microscope. In this example, the image of the resist is darker than the image of the wafer which has no resist. The line width measurement is therefore equivalent to the measurement of the width of the dark region of the image. However, in the imaging signal profile of the edge of an object such as the resist line, the transition from bright to dark is never perfectly sharp. Rather, it has been found that the signal profile generally exhibits a transition region between the dark (resist) area and the bright (silicon) area which may obscure and interfere with the determination of the exact dimensions. The problem that has been found is, in an imaging metrology system, how does one start with the image intensity profile and determine an accurate dimensional measurement.
Commercial equipment for IC pattern metrology has addressed this problem in several ways in the past. In one widely used technique, a signal threshold algorithm has been used to determine the pattern edge, e.g., the edge is assumed to be at a signal level half-way between the maximum and the minimum signal. Another algorithm might be to use the signal maximum, or the signal minimum, or the part of the profile with the steepest slope. The present invention describes a new algorithm for determining dimensional measurements based on imagining metrology signal profiles.
FIG. 1 illustrates a general block diagram of such a prior art system where an imaging system 20 creates an image of a test object 22 on a workpiece holder 24 which passes as imaging radiation 25 through lens 24 and is picked up by an image detector 26. The measurement apparatus includes a focus control system 32 to adjust the imaging optics (lens) 34 and the resulting imaging radiation 25. The image of the test object outputted by the image detector as an electronic image signal profile 28, which is analyzed by a signal analyzer 30 to obtain the final measurement results. The signal analyzer may also send the results to the focus control system for adjustment of the imaging optics.
A test object may contain, for example, two parallel lines which are to be measured. In an IC fabrication process, these lines might be thin metal patterns which are on a silicon wafer substrate. FIG. 2a shows a vertical cross section through test object lines 22a, 22b, cut perpendicular to the length of the lines. To measure the linewidth W of left line 22a, one must know the positions of the two edges, x.sub.1 and x.sub.2, whereby the width W is equal to x.sub.1 -x.sub.2. The center X.sub.left of the left line can be determined as X.sub.left =(x.sub.1 +x.sub.2)/2. And similarly the center of the right line is X.sub.right =(x.sub.3 +x.sub.4)/2. The distance between the centers of the two lines, which relates to overlay error measurements, can be given by the expression: EQU .DELTA.X=(x.sub.3 +x.sub.4 -x.sub.1 -x.sub.2)/2.
Linewidth measurement, overlay error measurement, and line center determination for wafer alignment are crucial in semiconductor process control. All of these measurements depend on precise determination of line edge position.
The imaging apparatus will acquire an image signal profile from which may be derived the line edge positions. A signal profile 28 is shown in FIG. 2b of test lines 22a, 22b from the view of FIG. 2a. Because of the finite resolution of the imaging optics, the signal profile does not have sharp edges, even though the assumed test object does. In the neighborhood of the line edge, the signal profile is sloped, and may have a more complicated structure. Because it is not apparent what point in the signal profile corresponds to an edge of the object, there is uncertainty in the dimensional measurement based on the object edges.
The most common method of determining edge positions from signal profiles is the threshold algorithm. In this technique, the signal profile is examined in the neighborhood of the assumed edge to determine a maximum signal level and a minimum signal level. The algorithm then assumes a threshold level chosen by the user, where 0% is the minimum and 100% is the maximum. The threshold algorithm locates the edge at the position where the signal level crosses the threshold level. This algorithm is illustrated in FIG. 2c for a typical choice of 50% threshold level. Since the edge position will depend on the threshold level, the user must make a careful choice. The edge positions determined by the threshold algorithm x'.sub.1, x'.sub.2, x'.sub.3, x'.sub.4 in FIG. 2c are not the same as the true edges x.sub.1, x.sub.2, x.sub.3, x.sub.4, respectively.
In addition to the threshold algorithm, other edge determination algorithms which have been used include 1) Minimum--where edge is assumed to be signal minimum; 2) Maximum--where edge is assumed to be signal maximum; and 3) Maximum slope--where edge is assumed to be at the point of the profile with the highest slope.
It is well known in the metrology field that current edge determination algorithms are not adequate. In the field of optical microscopy, D. Nyyssonen and R. Larrabee have shown in J. Res. Natl. Bur. Stds. Vol. 91, pp. 187-204 (1987) that the threshold value which corresponds to the true linewidth is highly dependent on the exact thickness of the patterned films, because of thin film interference effects. These effects greatly impact the accuracy of optical microscopy dimensional measurements of microstructures on wafers. In the field of SEM metrology, M. Rosenfield has shown in SPIE, Vol. 775, pp. 70-79 (1987) that the best threshold level varies from sample to sample, again impacting precision.
The problem of determining edges from signal profiles is an important one, and much attention has been devoted to it. Sophisticated methods of profile smoothing and statistical fitting techniques have been applied, but the basic algorithms are essentially the same as mentioned above. Accordingly, there is a long-felt need to improve the accuracy of edge determination and measurement in image analysis, especially relating to lithographically produced objects on semiconductors.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method of detecting and measuring edges of features using imaging apparatus whereby greater accuracy than has been attained in the past is achieved.
It is another object of the present invention to provide an improved method of detecting and measuring edges of features using available metrology imaging apparatus.
A further object of the invention is to provide an improved method of detecting and measuring edges of features which does not require additional parameters to be input, such as threshold values.
It is yet another object of the present invention to provide an improved method of detecting and measuring edges of features which results in smaller line width offsets and more tolerance to process variations.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.